1. Field of the Invention
The present invention relates to a time correcting apparatus using a standard time radio wave to correct a calculated current time and a radio controlled clock or watch provided with the time correcting apparatus.
2. Description of the Related Art
At present, in Japan, Germany, England and Switzerland, such services are provided that transmit a low frequency standard time radio wave(s) carrying time information from a transmitting station(s). In Japan, for instance, a transmitting station in Fukushima Prefecture transmits an amplitude modulated standard time radio wave of 40 kHz, and a transmitting station in Saga Prefecture transmits an amplitude modulated standard time radio wave of 60 kHz.
The standard time radio wave carries time information (TCO: time codes) including data of date: year, month, date, time and minutes, and is transmitted every 60 seconds, which means that a cycle of the time codes is 60 sec.
Clocks and watches (radio controlled clocks and watches) are widely used, which receive the standard time radio wave carrying the time codes, and detect the times codes from the standard time radio wave to correct a time to be displayed thereon. A receiving circuit of the radio controlled timepiece comprises a band pass filter (BPF), demodulating circuit, and processing circuit. The band pass filter receives radio waves from an antenna and detects only a standard time radio wave signal. The demodulating circuit demodulates the amplitude modulated standard time radio wave signal. The processing circuit reads the time codes included in the signal demodulated by the demodulating circuit.
A conventional processing circuit makes synchronization with a rising edge of a time code output signal (TCO signal, time codes) and measures a pulse wide of TCO signal, thereby obtaining a digital value (any one of P, 0, 1) corresponding to the detected pulse width, and obtains time information based on the obtained digital value.
In the conventional processing circuit, a second synchronization process, minute synchronization process, code reading process and matching judgment process are performed during a period from receipt of the standard time radio signal to detection of the time information. When an operation has not been properly performed in each process, the processing circuit has to perform the processes again from the first process. Therefore, noises included in the signal can disturb the operation of the process circuit, resulting in an extremely long time taken by the processing circuit before obtaining the time information.
The second synchronization process detects a marker or a position maker appearing every second in TCO signal. When the second synchronization process is repeatedly performed, a portion can be detected, in which a position maker PO locating at the tail end of frame and a maker M locating at the leading edge of frame continuously appear in the TCO signal. The portion appears every minute in the TCO signal. The position where the marker M appears is the leading position of the frame of TCO signal. An operation of detecting the portion where the position maker PO and the maker M continuously appear in TCO signal is referred to as “minute synchronization”. Since the leading edge of frame can be detected in the minute synchronization, an operation of reading codes starts thereafter. After one frame of data has been obtained, the matching judgment process is performed to judge a parity bit included in the data, thereby confirming whether the obtained data is impossible or not, that is, whether or not the obtained data indicates impossible year, month, date, time and minute. The minute synchronization is performed to detect the leading edge of frame. Therefore, the minute synchronization needs a period of 60 seconds to detect such leading edge of frame. Off course, searching for over plural frames of data, the minute synchronization takes more than 60 seconds to detect the leading edge of frame.
JP 2002-214372 A discloses a time adjusting apparatus which receives the standard time electromagnetic wave to detect time data, and stores the time data, and counts a periodical signal generated within the apparatus to obtain internal time data and corrects the internal time data using external data, and adjusts a frequency dividing value of the periodical signal based on a difference between the internal time data and the external time data. The periodical signal generated in the apparatus is corrected to enhance accuracy of the internal time data, whereby an interval of receiving the standard time electromagnetic wave can be made long.
In the technique disclosed in JP 2002-214372 A, the internal periodical signal is optimized, whereby a time can be displayed as precise as possible even when no standard time is received. But the process of receiving the standard time radio wave to obtain the external time data is substantially the same as conventional processes. And the technique still has a disadvantage that when an error should occur in the second synchronization process and/or minute synchronization process, the process has to be performed again from the beginning.